1. Field of the Invention
The present invention relates to a fuel cell stack.
2. Description of the Related Art
For example, a solid polymer fuel cell includes a unit cell in which separators hold therebetween an electrolyte membrane-electrode assembly (MEA) that includes an electrolyte membrane formed by a polymer ion exchange membrane and provided with an anode-side electrode and a cathode-side electrode on the opposite sides thereof. This type of fuel cell is normally used as an in-vehicle fuel cell stack and the like, with a predetermined number of the unit cells stacked.
This type of fuel cell stack needs to be applied with a favorable tightening load in the stacking direction to obtain desired power generation performance and exert the sealing function.
In view of the above, a fuel cell stack and tightening method thereof disclosed by Japanese Unexamined Patent Application Publication No. 2005-142042 is known, for example, which intends to obtain uniform pressure distribution in the entire surface of each of the unit cells. According to Japanese Unexamined Patent Application Publication No. 2005-142042, as illustrated in FIG. 6, an end plate 3a, fuel cell constituent members 4, and an end plate 3b are stacked in the vertical direction on a stacking table 2 forming a pressure device 1. The end plates 3a and 3b are temporarily tightened by a few tie-rod bolts 5 placed therebetween.
The pressure device 1 includes a plurality of pressure cylinders 7, the pressure force of which is adjusted by the pressure supplied by pressure force control devices 6. Each of the pressure cylinders 7 is provided with a pressing block 8 at the leading end thereof, and the pressing block 8 is provided with a load cell 9 on a pressing surface thereof.
Herein, after the respective pressure forces of the pressure cylinders 7 uniformly reach zero torque, the respective pressures of all of the pressure cylinders 7 start to be increased. Further, the respective pressure forces input by the load cells 9 are monitored. If all of the pressure forces reach a preset pressure force value, all of the pressure force control devices 6 stop increasing the pressure. Then, the tie-rod bolts 5 are sequentially tightened, and thereafter the pressure force applied by the pressure device 1 is released.
According to Japanese Unexamined Patent Application Publication No. 2005-142042, the respective pressure forces obtained from the load cells 9 are monitored to obtain a uniformly pressurized state in the surface of each of the cells. Thereafter, the tie-rod bolts 5 are tightened, while the pressure force of the pressure device 1 is released. Depending on the tightened state of the tie-rod bolts 5, therefore, the uniformly pressurized state may fail to be maintained in the entire surface of each of the cells.
Further, the fuel cell stack needs to be subjected to a further tightening process (increased tightening) in the stacking direction to compensate for a reduction in tightening load with the lapse of time of use. In this case, according to Japanese Unexamined Patent Application Publication No. 2005-142042, the fuel cell stack again needs to be set in the pressure device 1, subjected to the pressure force applying process by the pressure force control devices 6, and thereafter subjected to the tightening process using the tie-rod bolts 5.
As a result, an issue arises in that the pressure distribution in the surface of each of the cells tends to vary in the retightening process using the tie-rod bolts 5 and thus a high-performance and favorable increased tightening process fails to be carried out.